Conventionally, various vacuum capacitors (for example, Patent Document 1) have been used for impedance adjustment in the high-frequency power supply for normally used semiconductor facilities and a high-frequency apparatus of the large power oscillation circuit etc.
FIG. 6 is a schematic view for explaining an example of a typical vacuum capacitor. In FIG. 6, a reference sign 1 is a vacuum casing. The vacuum casing 1 is formed mainly from a tubular insulation member (e.g. a member made of ceramic material; hereinafter called an insulation tube) 2 and conductive members (e.g. members made of metal such as copper) 3, 4 which are provided on one end side and the other end side of the insulation tube 2.
As can be seen from the drawing, for instance, the conductive members 3, 4 are formed from metal cylinders 5, 6 which are provided on the one end side and the other end side of the insulation tube 2 and flanges (flanges on a movable electrode side and a fixed electrode side; hereinafter called a movable side flange and a fixed side flange) 7, 8 which are provided to close the insulation tube 2 and the metal cylinders 5, 6. The movable side flange 7 and the fixed side flange 8 could also be used as external terminals.
A reference sign 9 is a fixed electrode that is formed from a plurality of cylindrical electrode members whose inside diameters are different from each other. Each electrode member is provided concentrically on an inner side of the fixed side flange 8 (inside the vacuum casing 1) at a predetermined distance. A reference sign 10 is a movable electrode that is formed from a plurality of cylindrical electrode members whose inside diameters are different from each other, same as the fixed electrode 9. Each electrode member is provided concentrically at a certain distance. The movable electrode 10 is provided inside the vacuum casing 1 so that the each electrode member of the movable electrode 10 can be inserted into and extracted from a gap between the electrode members of the fixed electrode 9 with the each electrode member of the movable electrode 10 in noncontact with the electrode members of the fixed electrode 9. A reference sign 11 is a movable conductor. The movable conductor 11 is formed from a movable electrode supporting member 12 that supports the movable electrode 10 and a movable rod 13 that protrudes from a back surface (a surface on which the movable electrode 10 is not secured) of the movable electrode supporting member 12.
A reference sign 14 is bellows that are made of soft metal having elasticity (e.g. a member made of phosphor bronze or a member formed from a copper-clad SUS member), as a part of current path of the vacuum capacitor. One side edge of the bellows 14 is connected with a bearing portion 15 (by brazing) and the other side edge of the bellows 14 is connected with the movable conductor 11 (by brazing) so that the movable conductor 11 (the movable electrode 10) can be raised and lowered in an axial direction while a vacuum chamber 18 enclosed by the fixed electrode 9, the movable electrode 10 and the bellows 14 inside the vacuum capacitor being kept airtight.
With regard to the movable rod 13, for instance, a columnar member is used for the movable rod 13. And as mentioned above, the movable electrode 10 is fixed to one end side of the movable rod 13, and the other end side of the movable rod 13 is movably supported by the bearing portion 15 provided in the movable side conductive member 4. As a support structure of the movable conductor 11, such a structure that the other end side of the movable conductor 11 is screwed into a member which is rotatably supported by the bearing portion 15, could be employed. As an example, such a structure that a male screw part formed at the other end side of the movable conductor 11 is screwed into a female screw part of a nut member (a member for adjusting a position of the movable electrode; hereinafter called an adjustment nut) which is rotatably supported by the bearing portion 15, could also be employed.
By moving the movable conductor 11 in the axial direction and inserting and extracting the movable electrode 10 into and from the fixed electrode 9 (inserting and extracting the movable electrode 10 into and from the fixed electrode 9 so that the respective electrode members of the both electrodes 9, 10 alternate with each other), an area between facing electrodes (an overlap area between the fixed electrode 9 and the movable electrode 10) changes. With this, when voltage of the opposite polarity is applied to the both electrodes 9, 10 respectively and the area between facing electrodes changes, a value of capacitance appearing between the both electrodes 9, 10 is seamlessly changed, then the impedance adjustment is made.
Regarding high frequency current for the high-frequency apparatus of a case using such vacuum capacitor, the high frequency current flows from the movable side flange 7 to the fixed side flange 8 through the bellows 14 and the capacitance between the facing electrodes. Nowadays, a load used in the high-frequency apparatus becomes large, and the high frequency current increases with increase of the load. Thus the frequency with which frequent adjustment of the flow of the large current is performed has been high.
As one of characteristics required of such vacuum capacitor, it is to maintain a withstand voltage characteristic while maintaining the vacuum state of the vacuum chamber 18. On the other hand, for instance, as mentioned above, when current is applied to the movable electrode 10 and the fixed electrode 9, metal particles (suspended particles) and gas etc. appear in the vacuum chamber 18, and this might cause reduction of degree of vacuum of the vacuum chamber 18, and there arises a problem that the vacuum state cannot be maintained. Further, for instance, because trace molecules of hydrogen etc. have properties such that they permeate through material, there is a problem that the vacuum cannot be maintained for an indefinite term.
Therefore, a method in which a getter 16 is set in the vacuum chamber 18 and an adsorbed object (that is able to be adsorbed by the getter) such as the metal particles and the gas in the vacuum casing 1 is adsorbed by the getter 16, has been employed. As that example, a method using such an evaporation type getter (a dispersion getter) that a getter constituent is evaporated by the heating is known. The method is the following; the evaporated getter constituent adheres to an inner wall of the vacuum casing 1, and the adsorbed object in the vacuum chamber 18 is adsorbed by the adherent constituent. Or a method using a non-evaporation type getter (a junction getter; e.g., a getter made of substances such as magnesium, barium and aluminum that are apt to combine with gases) whose surface is protected by an oxide film is known. The method is the following; the oxide film is removed by the heating and the surface is activated, then the adsorbed object is adsorbed. Furthermore, a manner simultaneously employing the above both methods is known too.
In a case where the above getter 16 is merely set on a surface of the inner wall (e.g. an inner wall of the metal cylinder 6) in the vacuum casing 1, an adsorption effect becomes inadequate and the vacuum chamber 18 cannot be maintained at a high vacuum state, and this brings about a problem that causes an unstable withstand voltage characteristic. Considering this problem, recently, a method that sets the getter 16 on a surface of an inner wall of the fixed side flange 8 (inside the vacuum casing 1) around a center axis of the fixed electrode 9 or on a surface of the movable electrode supporting member 12 around a center axis of the movable electrode 10, has been employed (Patent Document 2).
However, in the vacuum capacitor used for the high-frequency power supply of the semiconductor manufacturing system and the oscillation circuit of a large power oscillator etc., when each electrode is supplied with electric charge, electric field appears around the electrodes and movable electrode supporting member. Because of this, in the case of the conventional method that merely sets the getter on the surface around the center axis of the movable electrode or the fixed electrode, there is a risk that voltage will be applied to the getter by the electric field (especially when such electric field distribution that the getter is a positive electrode side appears).
As described above, when the getter is affected by the electric field, a condition in which electrons reach the getter occurs, then, for example, kinetic energy of the electron is converted into thermal energy, a temperature of the getter therefore rises and reaches a re-release temperature, or electron stimulus (or impulse) occurs (transition of an electronic state occurs). For this reason, there is a possibility that the getter will be deteriorated (life of the getter will be shortened) and a desorption phenomenon of the adsorbed substance will occur.
As explained above, when the adsorbed substance, which has been adsorbed once by the getter, is re-released from the getter by the influence of the electric field, the degree of vacuum of the vacuum chamber is lowered, and the withstand voltage characteristic deteriorates.
The present invention is made for solving these problems, and an object of the present invention is to prevent the reduction of the degree of vacuum of the vacuum chamber and to maintain a desired withstand voltage characteristic, in the vacuum capacitor used for the high-frequency power supply circuit of the semiconductor manufacturing system and the oscillation circuit of the large power oscillator etc., in consideration of the electric field appearing when the current is applied to the vacuum capacitor.    Patent Document 1: Japanese Patent No. JP3264005 (paragraphs [0031]˜[0036], FIG. 1)    Patent Document 2: Japanese Patent Application Publication No. JP6-196363 (paragraphs [0016]˜[0023], FIG. 1)